Dinefwr passed to the English crown in 1276. A large round keep stands within a curtain wall, and a round tower in the north-west of the inner ward also survives relatively intact, while some of the residential buildings within the inner ward have been partly restored.
Dinefwr was the ancient capital of Deheubarth from the early 10th century, from the time of Hywell ap Cadell (Hywell the Good). Rhys ap Tewdwr (d. 1093), a descendent of Hywell, was the last true king of Deheubarth. Rhys's grandson, Rhys ap Gruffudd (Lord Rhys), took control of Dinefwr around 1160 and probably refortified the earlier fortress.
Lord Rhys's third son, Rhys Gryg, successfully challenged his elder brother Gruffudd for control of Dinefwr. Gruffudd's son Rhys Ieunc launched an unsuccessful attack on Dinefwr in 1213.
Shortly after, however, Llewelyn ap Iorwerth marched south to enforce the terms of the Peace of Worcester, which would have made the Princes of Deheubarth vassals of King Henry III of England. Rhys Gryg refused to submit, and destroyed his own ancestral seat at Dinefwr as an act of defiance.
In a remarkably quick turnabout, Rhys altered his stance only a few days later, and not only agreed to submit to Llewelyn, but became a vassal of Henry III and one of his staunchest supporters.
Sometime between 1220 and Rhys's death in 1234 Dinefwr was rebuilt, wit the main addition being a strong round keep at the east end of the inner bailey. A later hall stands on the north side of the inner ward.
After the Battle of Bosworth in 1483 Henry VII gave Dinefwr to one of his most trusted supporters, Sir Rhys ap Thomas. One of Thomas's descendants was responsible for the mock Gothic mansion of Newton House, visible amongst the trees in the valley below the castle.
The castle is reached via a footpath through the lovely landscaped park of Newton House, also known as Dinefwr Park.